1. Field of the Invention
The present invention relates to a charged-particle beam instrument adapted to inspect samples of workpieces, such as semiconductor devices (e.g., ICs and LSIs), treated in semiconductor fabrication processes, especially edge portions of samples.
2. Description of Related Art
In a fabrication process for fabricating semiconductor devices such as ICs and LSIs, each sample is inspected through observation of the substrate (such as a wafer) (hereinafter referred to as the sample) on which semiconductor devices are fabricated.
Such an inspection of samples is performed nondestructively, for example, by electron beam irradiation with a scanning electron microscope (SEM).
FIG. 1 schematically shows a scanning electron microscope performing an inspection of a sample. In the figure, the microscope has an electron optical microscope column 1 mounted to an upper part of the wall of a vacuum chamber 2.
Mounted inside the microscope column 1 are an electron gun 3, a condenser lens system 4 and an objective lens system 5 for focusing the electron beam from the electron gun onto the sample, deflectors 6 for scanning the beam from the gun 3 over the sample, and an axially symmetrical secondary electron detector 7 for detecting secondary electrons emanating from the sample. The deflectors 6 consist of X and Y deflectors.
A sample stage assembly 9 is mounted inside the vacuum chamber 2. An electrostatic chuck 8 is attached to the top surface of the stage assembly 9. A sample 10 is held to the chuck 8. The stage assembly 9 consists of an X stage and a Y stage for movements in the X- and Y-directions, respectively, such that the sample 10 can be moved within a plane perpendicular to the center axis O.
When a scanning signal is sent to the deflectors 6 from a scanning control circuit 12 that operates according to an instruction from a controller 11, the electron beam focused onto the sample 10 scans a desired area on the sample surface in two dimensions. This scanning induces secondary electrons from the sample 10. The secondary electrons are detected by the secondary electron detector 7. The output from the detector 7 is amplified by an amplifier 13, whose output signal is sent to the controller 11. The controller processes the input secondary electron signal and sends the resulting signal to a display device 14, where a secondary electron image of the sample is displayed.
An X-ray detector (not shown in FIG. 1) is mounted above the sample. When a desired area of the sample is irradiated with the electron beam, characteristic X-rays produced from the area are detected by the X-ray detector. The output signal from the detector is sent to the controller 11. The controller performs elemental analysis of the desired area based on the output signal from the detector which indicates the characteristic X-rays.
The human operator inspects the sample based on observation of the secondary electron image of the desired area or on the elemental analysis.
The inspection of the sample as described above is principally performed on sample surfaces. In recent years, however, there is an increasing necessity to inspect side surfaces, their vicinities (edge portions), and rear surfaces of samples, in addition to front surfaces, for the following reason. Silicon wafers which are typical semiconductor substrates have been increased successively in diameter taking account of cost reduction and other factors from 50 mm to 200 mm through 75 mm, 100 mm, 125 mm, and 150 mm. Recently, silicon wafers having a diameter of 300 mm have appeared. However, the thickness has hardly varied although the diameter has been increased. Therefore, if a scratch or cut is present at an edge of a wafer, the wafer itself easily breaks.
In some cases, particles adhere to edges of wafers. In such a case, as the fabrication process proceeds, the particles move and adhere to the mirror-finished surface where a lithographic pattern is formed. Alternatively, during a thermal process, the particles are removed by a thermal treatment and adhere as filmy matter to the mirror-finished surface.
If particles adhere to the rear surface of a wafer, the particles adhere to the wafer support member. As a result, the surface of a wafer supported next may be contaminated.
In any case, there is a danger that the wafer itself becomes defective. Accordingly, it is necessary to inspect whether there is any scratch, cut, or particles at the edges and on the rear surface of the silicon wafer.
First, an edge can be inspected by the following sequence. The stage assembly 9 is tilted, for example, as shown in FIG. 2 to tilt the sample 10 such that the edge is placed opposite to the microscope column 1. A mechanism for tilting a sample is shown, for example, in JP9017370. Alternatively, the microscope column itself may be tilted without tilting the sample. In a further feasible method, the sample is broken and a part 10′ including an edge is held to a holder H such that the edge is placed opposite to the microscope column 1 as shown in FIG. 3.
On the other hand, inspection of the rear surface of a sample is enabled by supporting the sample to an electrostatic chuck such that the rear surface is placed opposite to the microscope column.
However, in a method of tilting the sample or microscope column, it is difficult to inspect edge portions close to the rear surface of the sample or the rear surface itself.
Where the sample is broken as described above, the sample is destroyed. Therefore, in-line inspection cannot be achieved.
In the method where the sample is turned upside down and supported on a holder, in-line inspection cannot be performed because the sample surface having a lithographic pattern is damaged.